Refrigeration apparatus

ABSTRACT

Apparatus for refrigerating and heating comprising a source of liquid nitrogen arranged to deliver nitrogen selectively through coils of an evaporator or through heating apparatus and then through coils of the evaporator such that nitrogen can be employed for either cooling or heating a cargo compartment. Nitrogen gas exhausted from coils of the evaporator is released to ambient atmosphere outside of the compartment. Temperature sensors are positioned for sensing the temperature of nitrogen gas exhausted to atmosphere and to sense the temperature of air which has been drawn across the evaporator coils. Control apparatus initiates a defrost cycle to cause heated nitrogen vapor to be passed through evaporator coils for defrosting the coils when ice accumulates thereon in sufficient quantity to insulate the coils causing a predetermined temperature differential between the exhaust gas and air drawn across the evaporator coils.

United States Patent 9] 111 3,802,212

Martin et al. [451 Apr. 9, 1974 REFRIGERATION APPARATUS PrimaryExZlminer-Williaim J. Wye [75] Inventors: Patrick Martin; Barron Attorney, Agent, or Firm-Howard E. Moore; Gerald Moody, both of Dallas, Tex. Crmsmger [73] Assignee: General Cryogenics, Inc., Dallas, 57 ABSTRACT Tex.

Apparatus for refrigerating and heating comprising a [22] Fil d; May 5, 1972 source of liquid nitrogen arranged to deliver nitrogen selectively through coils of an evaporator or throu h [2] 1 Appl' 250616 heating apparatus and then through coils of the evapirator such that nitrogen can be employed for either 1521 U5. CI 62/52, 62/80, 62/81, l ng r ing a arg ompartment. Nitrogen gas 62/150, 62/151, 62/156, 62/275, 2/208, exhausted from coils of the evaporator is released to 62/514, 62/407, 165/2, 62/419, 62/ 159 ambient atmosphere outside of the compartment.

[51] Int. Cl. Fl7c 7/02 Temperature sensors are positioned for sensing the [58] Field of Search 62/80, 81, 150, 151, 156, temperature of nitrogen gas exhausted to atmosphere 62/52, 514, 275, 276 and to sense the temperature of air which has been drawn across the evaporator coils. Control apparatus [56] References Cited initiates a defrost cycle to cause heated nitrogen vapor UNITED STATES PATENTS to be passed through evaporator coils for defrosting the coils when ice accumulates thereon in sufficient 3,340,700 9/1967 Boise 62/52 3 374 640 3/1968 B0186 quantity to insulate the coils causing a predetermined temperature-differential between the exhaust gas and air drawn across the evaporator coils.

19 Claims, 9 Drawing Figures Dimnsw ewe L uzn COOL CNL' arr REFRIGERATION APPARATUS BACKGROUND OF INVENTION Apparatus has been devised heretofore for spraying vapor of cryogenic liquid through nozzles into an insulated compartment in which food products are transported. One such spray system is described in U. S. Pat. No. 3,525,235 which points out undesirable characteristics of such systems including variations of several degrees Fahrenheit in temperatures at various locations throughout a compartment.

Spray type systems which discharge cryogenic gas, such as nitrogen, into a refrigeration compartment have numerous inherent characteristics which render the use thereof undesirable because of the presence of the nitrogen enriched atmosphere within the compartment. Meats exposed tothe atmosphere often absorb the nitrogen resulting in discoloration of the meat. Vegetable products over a period of time wilt when deprived of oxygen.

The nitrogen gas deposited by a spray system must be removed from the compartment before products can be unloaded therefrom since workman cannot breathe the nitrogen enriched atmosphere. Such evacuation of the atmosphere within the-cargo compartment has required excessive quantities of nitrogen to cool the fresh air after a door has been opened to unload a portion of I the cargo. The fresh humid air also resulted in excessive frosting.

Refrigeration systems heretofore devised, wherein vapor from cryogenic gas was directed through coils of an evaporator through which air within a compartment was circulated by a fan, have been generally commercially unacceptable because excessive quantities of moisture collect on the evaporator coils forming ice, substantially reducing the heat transfer rate from the coolant to the air drawn across surfaces of the evaporator.

Refrigeration systems heretofore employed for cooling cargo compartments in which food products were transported did not have the capability of circulating heated air uniformly through the compartment when the ambient temperature outside the compartment was less than the minimum allowable temperature inside the compartment. Consequently transporting vegetables, such as lettuce and tomatoes, during winter months in northern portions of the United States and Canada required the use of space heaters positioned at various locations in the compartment to prevent freeze damage.

SUMMARY OF INVENTION We have developed apparatus for controlling temperature within a mobile compartment wherein vapor delivered from a source of liquid nitrogen or other suitable cryogenic gas, is directed through the evaporator coils for cooling same and then directed through a pneumatically operated motor for driving a fan to circulate air in the compartment across surfaces of the evaporator coils. Nitrogen vapor after passing through the evaporator coils and motor is exhausted to atmosphere.

Temperature sensitive control apparatus is employed for regulating the flow rate of nitrogen vapor through the evaporator coils. The control apparatus is adapted to divert nitrogen through a vaporizer, exposed to ambient temperature, and to direct vapor therefrom through a heating apparatus. The vapor is heated to a temperature of for example 1,000 F. and is delivered through the evaporator coils and the pneumatic motor to cause heated air to be circulated through the storage compartment.

Temperature sensing apparatus is disposed within the system for sensing the temperature differential between return air drawn from across the evaporator coils and nitrogen vapor exhausted from the evaporator coils and pneumatic motor to ambient atmosphere.

We have observed that the heat transfer rate through walls of the evaporator coils is high so long as the temperature differential between the exhaust coolant and air drawn across surfaces of the evaporator coils is less than approximately 35 F. However, as ice begins to form on surfaces of the evaporator coils, the coils become insulated reducing the heat transfer rate through the walls thereof which requires defrosting if efficient operation is to be maintained. Control apparatus is connected to the temperature sensors such that heated nitrogen vapor is routed through the evaporator coils and the pneumatic motor for defrosting same when the temperature differential between return air and exhaust gas exceeds a predetermined limit, for example 35 F.

The defrost cycle is terminated by the control apparatus when the temperature of the heated exhaust vapor and the temperature of the return air, which has 1 passed over surfaces of the evaporator coils, are substantially equal.

A primary object of our invention is to provide refrigeration apparatus particularly adapted to control temperature of a compartment in any vehicle, such as a transport trailer, railroad car, airplane or ship, which is self-contained and which utilizes liquified gas to refrigerate, heat and defrost a compartment without connection to an external source of power.

Another object of our invention is to provide refrigeration apparatus utilizing liquified gas to provide high refrigeration capacity without altering the normal oxygen content in the compartment.

A further object of the invention is to provide refn'geration apparatus which employs the expanding force of cryogenic gas to drive a pneumatic motor to provide forced air circulation through a compartment for providing even temperature control therethrough.

A still further object of our invention is to provide a refrigeration apparatus, having a cryogenic evaporator coil, adapted to be triggered to a defrost cycle when the temperature differential between gas exhausted from the evaporator coil and return air drawn across surfaces of the evaporator exceeds a first predetermined limit and which is triggered back into a cooling cycle when the temperature differential reaches a second predetermined limit.

A further object of our invention is to provide refrigeration apparatus for transport vehicles having a minimum number of moving parts to provide a reliable system requiring minimum maintenance.

A still further object of the invention is to provide a refrigeration system for transport vehicles having high cooling capacity which occupies minimum space within the cargo compartment.

Other and further objects of our invention will become apparent by reference to the detailed description hereinafter following and to the drawings annexed hereto.

DESCRIPTION OF DRAWING Drawings of a preferred embodiment of our invention are annexed hereto so that the invention will be better and more fully understood, in which:

FIG. I is a side elevational view of a transport vehicle having refrigeration apparatus installed thereon:

FIG. II is a cross-sectional view taken substantially along line II-Il of FIG. I;

FIG. III is a schematic diagram of the refrigeration apparatus;

FIG. IV is a partially sectionalized elevational view of a storage vessel for cryogenic gas;

FIG. V is an end view of the tank illustrated in FIG. IV;

FIG. VI is a cross-sectional view taken substantially along line VI-VI in FIG. V;

FIG. VII is a plan view of heat exchanger coils;

FIG. VIII is a side elevational view of the heat exchanger coils; and

FIG. IX is a diagrammatic illustration of the sensor arrangement employed for triggering the defrost cycle.

Numeral references are employed to designate like parts throughout the various figures of the drawing.

DESCRIPTION OF A PREFERRED-EMBODIMENT Referring to FIGS. I and II of the drawing, the numeral 1 generally designates a transport vehicle having a cargo compartment 2 disposed therein. The vehicle I may assume any desired configuration having top and bottom walls 3 and 4 connected across and upper and lower ends of side walls 5 and 6 and end walls 8 and 9 forming an insulated area on the inside thereof.

As will be hereinafter more fully described, one or more sources 10 and 11 of liquified cryogenic gas are provided for delivering coolant to an evaporator 12. In the particular embodiment of the invention iilustrated in FIG. II, the evaporator 12 is secured to an upper portion of front end wall 8 of the transport and is arranged to force cooled air through a plurality of air ducts l4a-14f of varying lengths such that cooled air is distributed uniformly throughout the compartment 2.

The source of cryogenic gaslt) and 11 is connected through a vaporizer 15, preferably disposed outside the refrigerated cargo area 2, to a heating device 16. Heated vapor from heating device 16 is delivered through coils of evaporator 12 for defrosting same and for causing heating air to be delivered through the cargo compartment.

As will be hereinafter more fully explained, a container 18 houses apparatus employed for controlling the flow of both hot and cold vapor through coils of evaporator 12 and an indicator 20 is connected to suitable temperature sensing means inside cargo compartment 2 for providing a visual indication of the temperature therein.

Referring to FIG. III of the drawing, the source of cryogenic gas 10 preferably comprises an insulated container having an outer shell 22 and an inner shell 24 spaced by a vacuum chamber 23, as will be hereafter more fully described in the description relating to FIGS. IV, V and VI of the drawing.

Container 10 contains liquid nitrogen 30 and the volume of container 10 above the liquid nitrogen is filled with nitrogen vapor. An insulated tube 32 is arranged to deliver liquid nitrogen through flow control valve 34 to a line 36 communicating with branch lines 38 and 40.

Branch line 38 is connected through valve 42 and line 44 with primary coils 46 of evaporator 12.

The flow passage through valve 42 is controlled by suitable actuating means 43 connected to a valve element in the body of the valve. Actuator 43 is preferably pneumatically operated having a movable element disposed therein such that a change in fluid pressure delivered thereto through line 50 causes the movable element to move thereby shifting the valve element for controlling flow through valve 42. Line 50 is connected through a three-way solenoid actuated valve 52 and line 54 to pneumatic temperature controller 56.

Temperature controller 56 is of conventional design comprising a capillary type temperature sensor 56a connected through line 56b to control apparatus of the controller 56. Pressurized fluid is delivered from controller 56 through solenoid actuated valve 43a to line 54 so long as sensor 56a is maintained at a temperature higher than that set on a thermostat in the controller. Controller 56 preferably has a visual indicator associated therewith to indicate the temperature of air in the cargo compartment 2. Controller 56 preferably has temperature recordingapparatus associated therewith (not shown) for plotting temperature in relation to time. Such instruments are commercially available from the Partlow Corporation of New I-Iartford,.N. Y..

Pneumatic temperature controller 56 is connected through line 58, pressure regulator 60 and line 62 to an upper portion of nitrogen tank 10 such that nitrogen vapor is delivered therethrough.

During a cooling cycle liquid nitrogen passes through branch line 38, valve 42, and line 44 to the coils of evaporator 12.

For defrosting coils of evaporator 12, liquid nitrogen passes through branch line 40 to vaporizer 64 which is exposed to ambient atmosphere outside of the cargo compartment 2 to provide sufficient heating to cause the nitrogen to vaporize. Vapor from vaporizer 64 passes through line 66 and solenoid actuated valve 68 to the heating device generally designated by numeral 16. Heated vapor passing from heating device 16 passes through line 70 to vaporizer 12 as will be hereinafter more fully described.

The heating device 16 comprises a burner 72 and a pilot light 74 connected through lines 73 and 75, respectively, to valve 76. A suitable fuel, such as propane, is delivered through line 78 from tank 80.

A conventional pilot generator 82 is connected through conductors 83 and 84 to the coil 85a of thermostatically controlled pilot single pole double throw relay 8S having a pole 85b and contacts 851: and 85d. Pilot generator 82 and pilot relay 85 are safety devices adapted to prevent opening of valve 76 for releasing fuel from burner 72 unless pilot light 72 has been previously ignited. If pilot generator 82 is heated by pilot 74 current is generated maintaining coil 85a of relay 85 in an energized condition causing a circuit to be broken through pole 85b and contact 85c.

An electrical ignition system is provided comprising an igniter plug 86- connected through plug wire 87 to an igniter system 88.

The igniter system 88 comprises an induction coil and a vibrator which is arranged to periodically make and break the primary circuit. The igniter system is connected through line 160 to a source of low voltage electricity as will be hereinafter more fully explained.

Line 160 is connected to the primary of the induction coil through an automatic switch which is actuated by the vibrator. When the vibrator opens the circuit, the induction coil generates a high voltage, delivered through plug wire 87, which is sufficient to cause a spark momentarily to jump the static spark points of igniter plug 86 for igniting fuel delivered through burner 72.

A heat sensitive switch 69, adapted to close when temperature is more than, for example, 200 F and to open when temperature is less than 190 F, is positioned adjacent burner 72. Switch 69 is disposed in the energizing circuit connected with solenoid actuated valve 68 through which vapor is delivered to the heating device 16. It should be readily apparent that switch 69 prevents actuation of valve 68 to an open position unless burner 74 has been ignited for heating vapor passing through heating device 16.

Heated vapor delivered from heating device 16 passes through line 70 to line 44 communicating with the entrance end of primary coils 46 of evaporator 12. Line 70 also communicates through solenoid actuated valve 90 with line 92 which is connected to the intake passage of a pneumatic motor 94. The outlet passage of motor 94 is connected through a line 96 to secondary coils 48 of evaporator 12, said secondary coils being connected through line 98 to muffler 100 through which nitrogen vapor is exhausted to atmosphere outside the cargo compartment 2 of the vehicle.

Heating device 16 has coils l5 and as illustrated in FIGS. VII and VIII which are preferably arranged such that the axes thereof are disposed perpendicularly. The outer coil 15 has an inlet end 15a, connected through valve 68 to pipe 66 and vaporizer 64, and an outlet end 15b adapted to connect with the inlet end 15a of the inner coil 15. Inner coil 15' has a discharge end 15b connected to pipe 70 which delivers heated vapor to evaporator 12.

Heat from burner 72 is transferred through walls of coils 15 and 15' for heating nitrogen vapor flowing therethrough. Vapor discharged from coil 15 is preferably heated to a temperature of approximately bl ,000 F. providing a high heating capacity while expending a relatively small volume of liquid nitrogen. The volume of the heated vapor is approximately 600 times the volume of liquid nitrogen delivered to vaporizer 64 from tank 10.

As best illustrated in FIGS. IV, V and VI, tank 10, comprising inner and outer shells 22 and 24 separated by evacuated space 23, has pipes extending through the walls thereof. Liquid nitrogen flows into and out of tank 10 through tube 10a connected to line 32 communicating with evaporator 12 and vaporizer 64. Vapor passes through tube 10b to temperature controller 56. A vacuum line 100 is sealed after space 23 has been evacuated.

10b and 10b of tube 10b.

Pneumatic motor 94 has a shaft 102 on which a fan blade 104 is mounted such that the flow of nitrogen vapor through pneumatic motor 94 causes fan blade 104 to rotate causing air within the cargo compartment 2 of vehicle 1 to pass across the primary coils 46 and the secondary coils 48 of evaporator 12.

Shaft 102 has a pulley 106 secured thereto about which a belt 108, in driving engagement with a pulley 1 10 on an alternator, extends. Alternator 1 12 is a standard AC generator of the type normally used in present day modern automobiles such as the Delco DELCO- TRON, the Ford Autolite and alternators manufactured by Motorola and Leech-Neville. The alternator 112 is connected to be driven by the pneumatic motor 94 and has a field terminal, an output terminal and a neutral terminal.

Alternator 112 is connected with a voltage regulator which serves to regulate the voltage and current. Regulator 120 has a field terminal connected to line 114, and an output terminal connected through line 122, rectifier 124 and line 126 to line 153, as will be hereinafter described for maintaining battery in a charged condition.

The control system illustrated in FIG. III generally comprises double pole-double throw relays R1, R2, and R3 actuated by signals delivered from temperature controller 56 and defrost controller 128.

When thermostat 56t of temperature controller 56 calls for cooling, indicator light G is illuminated and valve 52 is held open.

When defrost controller 128 calls for defrosting, indicator light A is illuminated and the coils of relays R1 and R2 are energized to turn on the burner 72, to permit opening of valve 68, and to open valve 43.

During heat and defrost cycles valve 43 is closed because line 50 is vented through valve 52.

Defrost controller 128 comprises a resistance bridge network arranged for energizing or de-energizing a relay which controls apparatus for triggering and terminating a defrost cycle.

The resistance bridge network of defrost controller 128 includes a first thermistor 136 mounted in vapor exhaust line 98 through which nitrogen vapor is exhausted to atmosphere. Thermistor 136 is a temperature-sensitive resistance, the resistance of which varies with temperature.

A second thermistor 138 is disposed inside cargo compartment 2 and is positioned such that air drawn across coils 46 and 48 of evaporator 12 flows across thermistor 138 when returning to the cargo compartment 2.

Thermistor 136 is connected through lines 136a and 13611 to defrost controller 128 and thermistor 138 is connected thereto through lines 138a and l38b.

Thermistors 136 and 138 are connected in a resistance bridge network such that when the resistance of thermistor 136 increases more than the resistance of thermistor 138, because ice has formed on coils 46 and 48 of evaporator 12, the resistance bridge will become unbalanced. Current then energizes the coil 12811 of the relay connected across the bridge network, causing the flow of liquid nitrogen to evaporator 12 to be stopped and causing heated nitrogen vapor to flow therethrough melting ice from the surface thereof.

it should be apparent that temperature sensor devices such as silicon diodes and thermocouples may be employed in lieu of thermistors 136 and 138 for sensing temperature and that changes in voltage or current may be employed to trigger defrost controller 128. It should be further apparent to persons skilled in the art that defrost controller 128 may include other and further devices for detecting an unbalanced condition in the bridge network. For example, we contemplate employment of a differential integrated circuit amplifier to detect the unbalance and to feed an output voltage to a phase-controlled pulse generator which delivers trigger pulses to a triac and, thus, controls current through lines 173 and 174 to initiate and terminate defrost cycles.

Current for actuating the control apparatus and the heating apparatus is delivered from battery 140 having a negative terminal connected toground and a positive terminal connected through a fuse 141 and conductor 142 to an ammeter 143. The ammeter 143 is connected through line 116 to alternator 112 and through a line 144 to terminals 145a and 145b of a double poledouble throw main power switch 145. V

The main power switch 145 has an off position wherein pole 145e is disconnected from contacts 145a and 145b, a cool only position wherein pole 145e engages contact 145a, and a cool and heat position wherein pole 145e engages contact 14512. [n the cool and heat position pole 145f of main switch 145 engages contact 145d allowing a circuit to be completed through contact 56 Rc of relay S6r of temperature controller 56 to the coil of relay R1 as will be hereinafter more fully explained. When the switch 145 is in the cool only position the contact 56 Re of temperature controller 56 is isolated from the control system.

Pole 1452 of the main switch 145 is connected through line 146 and line 147 to the pole 56Ra of the relay in temperature controller 56.

Pole 145e of main switch 145 is connected through line 146, line 148 and line 149 to terminal 150 of defrost controller 128. Terminal 150 of defrost controller 128 is connected to pole l28b of the relay in the defrost controller and is connected to lines 136b and 138k connected to the thermistors 136 and 138, respectively, in a bridge network.

Pole 145e of main switch 145 is connected through conductors 146, 148 and 151 to the pole Rla of relay R1.

The pole 145e of main switch 145 is connected through conductors 146 and 152 to the pole R2!) of relay R2, and through conductors 146 and 153 to the pole 85b of pilot relay 85.

From the foregoing it should be readily apparent that when pole 145e of main switch 145 engages the contact 145a or 14512 the positive terminal of battery 140 is connected to pole 85b of pilot relay 85, to pole R2b of relay R2, to pole Rla of relay R1, to pole 128b of defrost controller 128, and to pole 56Ra of temperature controller 56.

Remaining parts of the electrical control system will be hereinafter described in conjunction with the operation thereof.

OPERATION The operation and function of the apparatus hereinbefore described is as follows:

Poles 145e and 14Sf of main power switch 145 are moved to the cool and heat position for energizing the control circuit.

If the thermostat 56t of temperature controller 56 is calling for a cooling cycle, electrical current is directed from the positive terminal of battery to pole 56Ra of temperature controller 56, as hereinbefore described, and through contact 56Rb of the relay of temperature controller 56, conductor 170, contact R30 and pole R3a of relay R3, and conductor 173 to lamp G to provide-visual indication that cooling is required.

Unless a defrost cycle has been initiated, the positive terminal of battery 140 is connected through pole 128b of the relay in defrost controller 128 to contact 128d, and through line 172 to the coil of solenoid actuated valve 52, holding valve 52 open permitting flow of nitrogen vapor therethrough. Vapor from container 10 passes through line 62, regulator 60, line 58, valve 430, line 54, valve 52 and line 50 to pressurize actuator 43 for maintaining valve 42 in an open position.

Liquid nitrogen then flows through line 32, valve 34, line 36,.branch line 38, valve 42 and line 44 into the primary coils 46 of evaporator 12. The liquid nitrogen is at a temperature of approximately -320 F, and as heat is absorbed through the walls of primary coils 46 air adjacent thereto is cooled. Nitrogen from primary coils 46 passes through line 92 for driving pneumatic motor 94 causing fan 104 to circulate air across the primary and secondary coils. Nitrogen exhausted from motor 94 passes through line 96 to secondary coils 48 to absorb as much heat as possible before being exhausted through line 98 across thermistor 136 to ambient atmosphere through muffler 100.

It should be readily apparent that no nitrogen passes into the cargo compartment of the vehicle.

As ice forms on coils 46 and 48 of the evaporator 12, the rate of heat transfer through walls of the coils is reduced. When the temperature of air moving across the coils of the evaporator 12 reaches a predetermined temperature, for example, 28 F, above the temperature of nitrogen discharged across thermistor 136, the resistance bridge network in defrost controller 128 becomes unbalanced energizing the coil 128a, causing pole 128b to move out of engagement with contact 1280! and into engagement with contact 128s.

When the circuit through contact 128d is broken, the flow of current through conductor 172 and the coil of solenoid actuated valve 52 is stopped causing valve 52 to shift to a position stopping the flow of liquid nitrogen therethrough and causing line 50 connected to actuator 43 to be vented to atmosphere. This closes valve 42 stopping the flow of liquid nitrogen to coils of evaporator 12.

When an electrical circuit is completed through contact l28c of defrost controller 128, current flows through conductor 173 and conductor 174 to illuminate lamp A indicating that a defrost cycle has been initiated. Current flows from conductor 174 through conductor 175 to energize the coil of solenoid actuated valve 90 for opening valve 90.

Current is also directed through contact 1280, conductor 173, conductor 176, and conductor 177 for energizing the coil of relay R3. Current then flows from conductor 176 through contact R3f, pole R312 of relay R3, and conductor 178 to pole R2a. If pilot light 74 is burning, pilot generator 82 will maintain the coil 85a of pilot relay 85 in an energized condition which maintains the coil of relay R2 in a non-energized condition.

Current from the pole R2a of relay R2 thus passes through contact R20 and conductor 165 to energize the coil of relay R1. When the coil of relay R1 is energized current from pole Rla, connected to the positive terminal of battery 140 as hereinbefore described, passes through contact Rld and conductor 179 to heat sensitive switch 69. When the heat sensitive switch 69 is heated to a temperature, for example 200 F. the circuit is completed therethrough connecting current through conductor 180 to the coil of solenoid actuated valve 68 causing valve 68 to open.

When the coil of relay R1 is energized, the circuit is completed from pole Rlb through contact Rlf and through conductors 181 and 182 opening valve 76 causing fuel to flow from container 80 through line 78, valve 76, and line 73 to burner 72 for heating the coils l and of heating device 16.

Heated vapor flow through conduit 70, valve 90, and line 92 to motor 94 for melting any ice on the surface thereof. Heated gas also flows from conduit 70 through conduit 44 to primary coils 46 of evaporator 12. Vapor discharged from motor 94 passes through the secondary coils 48 and is exhausted to atmosphere.

When the temprature differential between sensors 136 and 138 reaches a predetermined limit, for example when the temperatures are substantially equal the defrost controller 128 causes current to flow through 128a switching the pole l28b out of engagement with contact 1280, de-energizing the coils of relays R1 and R3 and de-energizing the coil of solenoid actuated valve 52. When such occurs the burner is turned off, valve 68 is closed stopping the flow of nitrogen vapor to heating device 16, solenoid actuated valve 90 is closed blocking the bypass directly to motor 94.

As pole l28b of defrost controller 128 moves into engagement with contact 128d the coil of solenoid actuated valve 52 becomes energized opening said valve which causes valve 42 to be opened permitting flow of liquid nitrogen to the primary primary coils 46 of the evaporator 12.

It should be appreciated that the intense heat of vapor delivered from the heating device 16 results in very rapid melting of ice on surfaces of the coils 46 and 48 of evaporator 12 and on the surfaces of motor 94. Although motor 94 is running during the defrost cycle, the defrost cycle is so short that the cargo compartment is not heated appreciably.

When the thermostat 56t in temperature controller 56 calls for a heating cycle, a switch in the thermostat 56: is closed energizing the coil of relay 56R causing pole 56Ra to move out of engagement with contact 56Rb into engagement with contact 56Rc, and energizing the coil of solenoid actuated valve 43a causing valve 43a to close.

When pole 56Ra of temperature controller 56 engages contact 56Rc, current is directed through a conductor 185 through pole l45f of main power switch 145 through contact 145b, and through conductor 186 to illuminate lamp R. Current is also directed from contact 145d to main power switch 145 through conductor 187, contact R3e and pole R3); of relay R3, and through conductor 178 to pole R2a of relay R2 for energizing the coil of relay R1 to light the burner 72, and to direct nitrogen vapor through valve 68, as hereinbefore described in the discussion related to the defrost cycle.

From the foregoing it should be readily apparent that we have developed apparatus employing liquid nitrogen, or other suitable cryogenic material, for cooling, heating and defrosting. The control system is selfsustained, being energized by a battery which is maintained in a charged condition by alternator 112 driven by motor 94 which is driven by the flow of nitrogen therethrough.

The system is completely automatic employing thermostat control means to initiate cooling and heating cycles and employing means for sensing a temperature differential between air in the cargo compartment and nitrogen vapor discharged from the evaporator coils for initiating and terminating defrost cycles.

It should be appreciated that other and further embodiments of our invention may be devised without departing from the basic concept of the invention.

Having described our invention, we claim:

1. A method of controlling the heat transfer rate through a wall of a tube comprising the steps of: delivering coolant through the tube; moving fluid in heat exchange relation with the tube; sensing temperature of coolant exhausted from the tube; sensing temperature of fluid which is passed in heat exchange relation to the tube; and heating surfaces of the tube when a predetermined differential between temperature of coolant exhausted and temperature of fluid which has moved across the tube exists.

2. The method of claim 1 wherein thesteps of sensing temperature of coolant exhausted and sensing temperature of fluid are accomplished by positioning a first temperature sensor in heat exchange relation with coolant exhausted from the tube; positioning a second temperature sensor in heat exchange relation with fluid which is passed in heat exchange relation with the tube, said first and second temperature sensors being connected in a resistance bridge network adapted to initiate the steps of heating surfaces of the tube and delivering coolant through the tube.

3. The method of claim 1 wherein the step of heating surfaces of the tube comprises, heating a volume of coolant; and delivering the heated coolant through the tube.

4. The method of claim 1 wherein the step of delivering coolant through the tube comprises, delivering liquified cryogenic gas through the tube.

5. The method of claim 4 wherein the cryogenic gas is nitrogen.

6. The method of claim 4 wherein the step of moving fluid in heat exchange relation with the tube comprises, delivering liquified cryogenic gas, which is circulated through the tube, through a means for driving a fan disposed in driving relation with the fluid.

7. A method of controlling apparatus to maintain temperature in a compartment within limits comprising the steps of: moving coolant through heat exchanger apparatus; moving fluid in heat exchange relation with the heat exchanger apparatus; generating a first signal representative of temperature of fluid in the compartment; generating a second signal representative of temperature of coolant exhausted from the heat exchanger apparatus; combining the first and second signals to produce a control signal; delivering coolant through the heat exchanger apparatus responsive to said control signal when the magnitude thereof is less than a predetermined value; and delivering heated fluid through the heat exchanger apparatus responsive to a control signal exceeding a predetermined value.

8. A method of controlling temperature in a compartment comprising the steps of: circulating liquid nitrogen through a primary coil; circulating nitrogen from the primary coil through a pneumatic motor arranged to drive a fan; circulating nitrogen from the motor through a secondary coil, said primary and secondary coils being positioned such that the motor moves air thereacross; and exhausting nitrogen from the secondary coil outside the compartment.

9. The method of claim 8 with the addition of the step of, stopping flow of liquid nitrogen to the primary coil when a predetermined quantity of ice has formed on surfaces thereof; delivering liquid nitrogen to a heating apparatus; directing heated nitrogen vapor through the primary coils, through the motor, and through the secondary coil for melting ice on surfaces thereof.

10. Temperature control apparatus comprising, a coil; means to deliver fluid through said coil; means to move air across the coil; first sensor means to sense temperature of air moving across the coil; second sensor means to sense the temperature of fluid exhausted from the coil; means to generate a signal when the temperature of air drawn from across the coil exceeds the temperature of fluid exhausted from said coil by a predetermined temperature; and means energized by said signal to heat surfaces of the coil to melt ice thereon.

11. The combination called for in claim 10 wherein the first and second sensor means each comprises temperature-sensitive resistance means positioned in heat exchange relation with the air and the coolant, respectively; and wherein the means to generate a signal comprises a resistance bridge network having the first and second sensor means disposed therein such that the bridge becomes unbalanced to generate a signal when the difference in temperature of airdrawn across the coil exceeds the temperature of fluid exhausted from the coil by a predetermined amount.

12. The combination called for in claim 10 wherein the means energized by said signal to heat surface of the coil comprises, heater means; signal responsive valve means arranged to deliver coolant to the heating device; and means to deliver heated coolant from the heating device to the coil.

13. The combination called for in claim 10 wherein the means to move air across the coil comprises, a fluid driven motor connected in driving relation with impellar means; and means to direct fluid from said coil through the motor.

14. The combination called for in claim 10 wherein the means to deliver fluid through the coil comprises,

15. The combination called for in claim 14 wherein the container comprises an outer shell; an inner shell disposed inside said outer shell; said shells being spaced apart to form a vacuum chamber between adjacent surfaces thereof; a first pipe segment extending through the wall of the inner shell; a second pipe segment extending through the wall of the outer shell, said first and second pipe segments having ends terminating in spaced apart relation in said vacuum chamber; and an insulated coupling arranged to join ends of said first and second pipe segments, said coupling being disposed in the vacuum chamber.

16. Apparatus to control temperature in a cargo compartment of a trailer comprising, a source of liquefied cryogenic gas carried by the trailer; an evaporator in the compartment; first conduit means connecting the evaporator and the source of liquefied gas; a heat exchanger; second conduit means connecting the heat exchanger with the source of liquefied gas; a pneumaticallyoperated motor driven fan arranged to cause air in the compartment to circulate over surfaces of the evaporator; flow control means having a first position wherein the source of liquefied gas is connected with the motor, and having a second position wherein the heat exchanger is connected with the motor and the evaporator; a burner arranged to heat said heat exchanger; a source of fuel; and supply conduit means between said burner and said source of fuel. 7

17. The combination called for in claim 16 wherein the heat exchanger comprises an outer coil of conduit and an inner coil of conduit, said outer coil being positioned about said inner coil, said inner 'coil having a central axis disposed transversely of a central axis of said outer coil.

18. The combination called for in claim 16 with the addition of a current responsive valve in said supply conduit means; a pilot burner adjacent said burner; a

' fuel supply line between said source of fuel and said pilot burner; heat sensitive switch means adjacent said pilot burner; and electrical conductors between said switch means and said current responsive valve to prevent opening of said valve unless temperature adjacent said pilot burner exceeds a predetermined temperature. I

19. Apparatus to control temperature in a compartment comprising, a source of liquefied cryogenic gas; a primary evaporator in the compartment; first conduit means connecting the primary evaporator and the source of liquefied gas; a pneumatically driven motor in the compartment; second conduit means between said primary evaporator and said motor; a secondary evaporator in said compartment; third conduit means connecting said motor and said secondary evaporator; a fan driven by said motor; and means supporting said primary and secondary evaporators such that the fan moves air across each of the evaporators. 

1. A method of controlling the heat transfer rate through a wall of a tube comprising the steps of: delivering coolant through the tube; moving fluid in heat exchange relation with the tube; sensing temperature of coolant exhausted from the tube; sensing temperature of fluid which is passed in heat exchange relation to the tube; and heating surfaces of the tube when a predetermined differential between temperature of coolant exhausted and temperature of fluid which has moved across the tube exists.
 2. The method of claim 1 wherein the steps of sensing temperature of coolant exhausted and sensing temperature of fluid are accomplished by positioning a first temperature sensor in heat exchange relation with coolant exhausted from the tube; positioning a second temperature sensor in heat exchange relation with fluid which is passed in heat exchange relation with the tube, said first and second temperature sensors being connected in a resistance bridge network adapted to initiate the steps of heating surfaces of the tube and delivering coolant through the tube.
 3. The method of claim 1 wherein the step of heating surfaces of the tube comprises, heating a volume of coolant; and delivering the heated coolant through the tube.
 4. The method of claim 1 wherein the step of delivering coolant through the tube comprises, delivering liquified cryogenic gas through the tube.
 5. The method of claim 4 wherein the cryogenic gas is nitrogen.
 6. The method of claim 4 wherein the step of moving fluid in heat exchange relation with the tube comprises, delivering liquified cryogenic gas, which is circulated through the tube, through a means for driving a fan disposed in driving relation with the fluid.
 7. A method of controlling apparatus to maintain temperature in a compartment within limits comprising the steps of: moving coolant through heat exchanger apparatus; moving fluid in heat exchange relation with the heat exchanger apparatus; generating a first signal representative of temperature of fluid in the compartment; generating a second signal representative of temperature of coolant exhausted from the heat exchanger apparatus; combining the first and second signals to produce a control signal; delivering coolant through the heat exchanger apparatus responsive to said control signal when the magnitude thereof is less than a predetermined value; and delivering heated fluid through the heat exchanger apparatus responsive to a control signal exceeding a predetermined value.
 8. A method of controlling temperature in a compartment comprising the steps of: circulating liquid nitrogen through a primary coil; circulating nitrogen from the primary coil through a pneumatic motor arranged to drive a fan; circulating nitrogen from the motor through a secondary coil, said primary and secondary coils being positioned such that the motor moves air thereacross; and exhausting nitrogen from the secondary coil outside the compartment.
 9. The method of Claim 8 with the addition of the step of, stopping flow of liquid nitrogen to the primary coil when a predetermined quantity of ice has formed on surfaces thereof; delivering liquid nitrogen to a heating apparatus; directing heated nitrogen vapor through the primary coils, through the motor, and through the secondary coil for melting ice on surfaces thereof.
 10. Temperature control apparatus comprising, a coil; means to deliver fluid through said coil; means to move air across the coil; first sensor means to sense temperature of air moving across the coil; second sensor means to sense the temperature of fluid exhausted from the coil; means to generate a signal when the temperature of air drawn from across the coil exceeds the temperature of fluid exhausted from said coil by a predetermined temperature; and means energized by said signal to heat surfaces of the coil to melt ice thereon.
 11. The combination called for in claim 10 wherein the first and second sensor means each comprises temperature-sensitive resistance means positioned in heat exchange relation with the air and the coolant, respectively; and wherein the means to generate a signal comprises a resistance bridge network having the first and second sensor means disposed therein such that the bridge becomes unbalanced to generate a signal when the difference in temperature of air drawn across the coil exceeds the temperature of fluid exhausted from the coil by a predetermined amount.
 12. The combination called for in claim 10 wherein the means energized by said signal to heat surface of the coil comprises, heater means; signal responsive valve means arranged to deliver coolant to the heating device; and means to deliver heated coolant from the heating device to the coil.
 13. The combination called for in claim 10 wherein the means to move air across the coil comprises, a fluid driven motor connected in driving relation with impellar means; and means to direct fluid from said coil through the motor.
 14. The combination called for in claim 10 wherein the means to deliver fluid through the coil comprises, a container; conduit means connected between said container and the coil; and means in said conduit means for controlling the flow of coolant therethrough.
 15. The combination called for in claim 14 wherein the container comprises an outer shell; an inner shell disposed inside said outer shell; said shells being spaced apart to form a vacuum chamber between adjacent surfaces thereof; a first pipe segment extending through the wall of the inner shell; a second pipe segment extending through the wall of the outer shell, said first and second pipe segments having ends terminating in spaced apart relation in said vacuum chamber; and an insulated coupling arranged to join ends of said first and second pipe segments, said coupling being disposed in the vacuum chamber.
 16. Apparatus to control temperature in a cargo compartment of a trailer comprising, a source of liquefied cryogenic gas carried by the trailer; an evaporator in the compartment; first conduit means connecting the evaporator and the source of liquefied gas; a heat exchanger; second conduit means connecting the heat exchanger with the source of liquefied gas; a pneumatically operated motor driven fan arranged to cause air in the compartment to circulate over surfaces of the evaporator; flow control means having a first position wherein the source of liquefied gas is connected with the motor, and having a second position wherein the heat exchanger is connected with the motor and the evaporator; a burner arranged to heat said heat exchanger; a source of fuel; and supply conduit means between said burner and said source of fuel.
 17. The combination called for in claim 16 wherein the heat exchanger comprises an outer coil of conduit and an inner coil of conduit, said outer coil being positioned about said inner coil, said inner coil having a central axis disposed transversely of a central axis of said outer coil.
 18. The combinatioN called for in claim 16 with the addition of a current responsive valve in said supply conduit means; a pilot burner adjacent said burner; a fuel supply line between said source of fuel and said pilot burner; heat sensitive switch means adjacent said pilot burner; and electrical conductors between said switch means and said current responsive valve to prevent opening of said valve unless temperature adjacent said pilot burner exceeds a predetermined temperature.
 19. Apparatus to control temperature in a compartment comprising, a source of liquefied cryogenic gas; a primary evaporator in the compartment; first conduit means connecting the primary evaporator and the source of liquefied gas; a pneumatically driven motor in the compartment; second conduit means between said primary evaporator and said motor; a secondary evaporator in said compartment; third conduit means connecting said motor and said secondary evaporator; a fan driven by said motor; and means supporting said primary and secondary evaporators such that the fan moves air across each of the evaporators. 